Injection molding machines are used for producing moldings made, for example, of thermoplastic material. The raw material, the plastic pellets, is melted in a heating cylinder by means of a plasticizing screw and is extruded into a mold by axial or longitudinal travel of the plasticizing screw. The mold is the cavity of a tool that is usually in two parts and is pressed together with the closing force by a moving tool clamping plate and fixed tool clamping plate. For removing the molding, the moving mold half is driven into a mold-open position and the part is removed from the cavity by an ejector. This procedure is repeated cyclically. Movement of the tool and of the ejector is not a process-controlled movement as such and, for the sake of productivity, should be very rapid, in other words should be carried out in the minimum amount of time. All these non-process dependent movements are combined in the so-called dry cycle time. Typical dry cycle times of a modern injection molding machine having a closing force of 100 t and 1.4 to 2.9 seconds. The dry cycle time is one of the main factors limiting productivity.
Nowadays, injection molding machines are still usually driven hydraulically using oil. Hydraulic drives have the advantage of being able to convert energy easily in a controlled manner into linear movements and high forces (electric motor.fwdarw.pump.fwdarw.valve system hydraulic cylinder). The relatively poor overall efficiency, which is about 20 to 40% in the case of injection molding machines, depending on the load, is a drawback. The lost power is usually absorbed by oil and water coolers. Hydraulic apparatus using oil do have high noise levels. Leakages, present to a greater or lesser extent in all oil hydraulic apparatus, are a further problem. Hydraulic oils are based almost exclusively on mineral oils and are therefore a hazard for the environment. Although the period of use of the hydraulic oils can be several years in normal cases, the problem of disposal arises.
Injection molding machines that are driven by an electric servomotor have been proposed in recent years. They are distinguished by much lower energy consumption but could not generally be used worldwide. A mechanical design has to produce a linear movement from the rotating movement of the servomotor, for example, up to the injection molding machine. Experience with machine tools has revealed two methods of converting the rotational movement of an electric motor into a longitudinal movement: the threaded spindle and the toothed rack. In the case of completely electric injection molding machines, only the toggle lever can be considered as a basic concept for the clamping unit. This is due to the conversion of the engine torque into the closing force. A force ratio of up to a factor of 50 is possible, depending on the design of the toggle joint geometry. A ball spindle preceded by a gear, for example, serves for driving the cross head. Such machines are designed for a dry cycle time that is as short as possible. However, practical values do not yet attain the corresponding peak values of hydraulically driven machines.
During the carriage movement, it is necessary to control the nozzle contact force in particular. The key problem in injection molding is injection has to take place at a controlled velocity, at a high pressure, and at the correct time. The plasticizing screw can attain an injection pressure of up to 2,000 bar or higher. An important object is the precise positioning in particular of the plasticizing screw. During injection, the velocity of the screw has to be controlled during the filling phase. On the other hand, the dwell pressure phase necessitates a controlled injection pressure or dwell pressure. The corresponding actual value originates from one or more measuring devices. Contrary to expectation, however, it has not been possible hitherto with electric drives to control the actual critical phases maintaining accurate back pressure during plasticisation and adequately control the melt pressure during the pack and dwell pressure phases. A large number of proposals have been made in recent times. Attempts have been made to control the injected quantity (melt flow) and the injection pressure by using various detector pulses, for example, for positioning the spindle or the plasticizing screw (see EP PS No. 216 940, 217 963, 167 631, 249 641). With that system, the velocity or acceleration parameters are subjected to open- or closed-loop control, in particular by the closed- or open-loop control of the torque of the driving motor. Attempts have been made to smooth out all deviations or the resultant errors stepwise by a plurality of special correction procedures using an error register (EP-PS No. 280 734). Similarly to hydraulic control or motion models, this process is based on the driving torque for controlling the injection force or the injection pressure. From a physical point of view, a more or less direct connection between the torque signal to the drive (closed- and open-loop motor controller) and the resultant or attainable injection pressure.
However, the corresponding processes have great drawbacks. Open/closed loop force control can in fact be achieved only in principle via the analog motor current limiting input at the drives (electronic controller to the servomotor). However, the following marginal conditions stand in the way of more precise or exact force control according to predetermined set values:
Firstly: the static actual force is, among other things, substantially impaired by frictional forces which are difficult to anticipate. These distort the torque signal over the path beginning with torque conversion of the motor current via the gear to forces or pressures.
Secondly: in contrast to a hydraulic system, inertia forces have a much more serious effect, among other things, during the changeover from filling pressure to dwell pressure. Together with the resilient plastic melt in the injection cylinder, these moved masses form low frequency mass/spring resonators. This makes it difficult and often impossible to produce precise or thin-walled parts. The control of all parameters, for example, during injection and plasticisation but particularly in the transition phases from filling to the dwell pressure (pack) and from the dwell pressure to plasticisation, limits the quality of the injection molding.
The development in control in the past has gone through various phases. Until the late 80s, the use of a process computer that coordinated all functions as a control center and in particular, subjected all main functions to open- and closed-loop control, represented the highest state. This model was superseded by the so-called memory programmable controllers (SPS) to which, however, a process computer was allocated for more complex computer tasks. The SPS was allocated the control and locking functions, but sometimes also start programs, run-up programs etc, and the process computer took over the actual closed- and open-loop control function.
Servomotors have been used increasingly widely for some time, the motor-generated rotational movement being transmitted as such or being converted into a translational movement or being converted into a transitional movement if necessary. The main advantage with servomotors is that both the position and the velocity of the axis can be controlled according to predetermined set values by a so-called interpolator with surprisingly high precision via the checking of the electric field (.phi.) and current control (I) or with corresponding moment control for the motor axis. A drawback, however, is that relatively complex intelligence which may be controlled only by specialized firms is required for the necessary control tasks. The structure for the open- and closed-loop control of the servomotors in the drive has to be made up with specially developed microelectronics owing to the corresponding complexity.
The structural unit comprising the velocity controller and, in particular, the current or torque actuator as well as the corresponding commutator function is described as a motor controller which, together with mains connection and various converters, represents an individual drive. It is important for the drive to have a current controller.
Nowadays, a complete machine controller, for example, for an electrically driven injection molding machining, usually has a so-called CNC controller and an interpolator arranged in the controller. The interpolator is a computer for the drive with considerably higher computing power. Depending on their function, the servomotors are distributed over the entire machine and are preferably designed as velocity and/or torque controllable electric motors. The drives may be combined as a group. Either frequencial or analog data transmission takes place, in particular for velocity control, from the interpolator via the necessary wiring to each drive. The corresponding signal is transmitted during analog transmission, for example in the form of a voltage having a value in the range of +/- (plus or minus) 10 volts.)
The actual drawback of this solution is that data transmission as such represents a problem area as signal lines, in particular for control tasks, have to be specially protected against interference fields. The very advantageous bus system cannot be used or can only be used to a very limited extent as the data transmission rate is no longer guaranteed with the bus system. Integration of the interpolator into the CNC controller is currently the best solution as it is acknowledged that the systems as a whole are exhausted to their power limit. A further increase in the controllability of the process could only be achieved with excessive use.